The field of this invention is radar and RF communications, in particular specifying the orientation of a highly directional airborne antenna.
In the field of radar and highly directional RF communications antennas, it is necessary to calibrate the angular orientation and position of an antenna in order to use the output of the antenna to specify the azimuth and elevation of the object that the antenna points to. The angular orientation of the antenna (usually the centerline of a parabolic dish reflector) is called the Line-of-Sight (LOS) or RF axis of the antenna. The antenna LOS is the direction of maximum transmitted and received RF energy. Ideally, as shown in FIG. 1, a local antenna (110) LOS is pointed at a remote antenna (200) along a straight line connecting the two antennas. The straight line connecting two antennas is also called the LOS. High frequency RF, radar, and light essentially travel in a straight line.
For example, an antenna mounted on a local airborne (A/B) platform 110 or aircraft (A/C) points to a remote target aircraft 200. In order to point the local antenna LOS at the remote target A/C antenna, the local antenna control computer must have the following information: a) local A/C position (latitude, longitude, and altitude) from the local navigator computer and remote A/C position from the remote navigator; and b) the orientation (roll, pitch, and heading shown in FIG. 2) of the local A/C from the local navigator. This navigational information is used to calculate (specify) and command azimuth and elevation antenna gimbal angles, with respect to the local A/C, to point at the remote target as shown in FIG. 6. FIG. 6 illustrates a portion of a computer system 100, in which antenna control computer 50 sends signals to the antenna gimbal servos 70, receiving input from keyboard 55 and from INS 60. Box 80 represents other portions of a system, such as disk drives, memory for storing software according to the invention, CRTs, other computers processing other input data (e.g. data indicating the general location of a potential target, etc.). In the example of target acquisition, information on the general location of a potential target may come from a coarse antenna on board the A/C or from a remote source, such as an AWACs plane. The antenna computer takes the INS data from system 60 and commands the antenna system 70 to sweep over the approximate location until acquisition occurs. After acquisition of the remote A/C, it will send location data over the data link. The signals sent to the antenna (and received from the antenna and transmitted to other parts of the system) will be corrected by bias values determined in the inventive procedure.
The antenna azimuth and elevation gimbals with resolvers (for sending measurements of the actual pointing direction of the antenna), in an xyz frame, determine the antenna coordinate system. This system must be aligned with the A/C navigator X1 roll, Y1 pitch, and Z1 heading coordinates as shown in FIG. 3 illustrating the relationship of the coordinate systems. Alignment of xyz to X1 Y1 Z1 is called xe2x80x9cLOS boresightingxe2x80x9d. Alignment can be achieved either mechanically or with computer software. This invention presents a software method.
In the past, the coordinates were mechanically measured in a lengthy process involving surveyor""s methods using theodolites, tape measures, and aligned with mechanical shims to adjust the azimuth and elevation orientations of the antenna to match the A/C navigator roll, pitch, and heading coordinates. Most methods involving survey theodolites, tapes, and shims have been very time consuming.
Highly directional airborne antennas can be pointed with an accuracy of about +/xe2x88x920.1 degree when referenced to the vehicle""s Inertial Navigation System (INS). The art has felt a need for a quick, economical method of boresighting consistent with this accuracy.
The invention relates to a method of boresighting an antenna mounted to an aircraft in which the Global Positioning System (GPS) is used to provide input to the boresighting process.
A feature of the invention is a set of measurements of a cooperative target RF source that points the A/B antenna LOS at a known location in space.
Another feature of the invention is the software calculation of adjustment parameters that adjusts radar and RF LOS output data without physically changing the orientation of the antenna.